1. Field of the Invention
The present invention concerns a method and an apparatus for generation of an x-ray image of an examination subject.
2. Description of the Prior Art
The diagnostic utilization capability of an x-ray image, meaning the ability to detect structures within the examination subject, is significantly influenced by two factors. These are apparatus-dependent acquisition parameters such as, for example, the anode/filter combination of the x-ray tube, and the correct exposure, and the manner with which the x-ray image is displayed on a reproduction medium.
For exposure or dose control, i.e. to control the x-ray dose required for an x-ray image, it is known (in particular in the implementation of diagnostic or therapeutic methods) to acquire a first, complete x-ray image with a dose estimated based on experiential values (which x-ray image is diagnostically usable), for example given the image support of an operative procedure in which it is necessary to generate a number of x-ray images of the examination subject in short time intervals. In a measurement field of this x-ray image that is situated in the central image region, the arithmetic average of the intensity or of the brightness is then calculated, for example. This average is compared with a stored desired value. By means of this comparison the dose for the acquisition of the next x-ray image is adjusted such that the real value of the intensity or brightness coincides optimally well with the desired value.
The measurement field used for the determination of the real value is static with regard to its position, shape and size, meaning that it is always the same in all exposures. Such a static measurement field, however, can lead to a reduced image quality in disadvantageous cases due to a non-optimal x-ray dose. One of the main causes for this is direct radiation incident in the measurement field. Direct radiation is x-rays that have not passed through the examination subject and thus are unattenuated. Due to this direct radiation the real value of the intensity averaged over the measurement field is raised. This leads to the situation that the x-ray dose for the following acquisition is reduced until the desired value is reached. The result is an underexposed x-ray image.
A cause for direct radiation in the measurement field can be, for example, a poor positioning of the examination subject during the imaging or the fact that the examination subject being examined is smaller than the actual, statically-defined measurement field.
For correct use of the dose control, in particular in mobile C-arm x-ray apparatuses, it is therefore necessary for the user (the doctor or medical-technical assisting personnel) to position the patient (i.e. the examination subject) such that optimally no direct radiation can strike the central measurement field. In other words; the central measurement field must be optimally completely covered by the patient. Experience shows, however, that such an ideal positioning of the patient is not always ensured or possible for the imaging. In order nevertheless to obtain qualitatively good x-ray images in such cases, in principle the possibility exists to deactivate the automatic dose controller and to manually control the acquisition parameters, but this does not represent a satisfactory solution in light of experience.
In addition to an optimal dose control, in the reproduction of x-ray images it is also desired that the image regions that are diagnostically relevant for the doctor be shown with optimal quality on the monitor or in the archiving as a hard copy (film, foil). This diagnostically-relevant image region is formed by the region of the image field in which the image of the subject (the patient) is located.
Optimal quality means that the grayscale values within the subject image region enable an optimally differentiated, high-contrast (and thus well recognizable and thus diagnostically usable) reproduction of structures within this subject image region. By contrast, the remaining image regions should be reproduced such that they do not distract the observer. A main problem in the reproduction of the x-ray images is the direct radiation regions that occur in the x-ray image with high brightness.
The image data that are present at the output of an x-ray receiver and already subjected to a pre-processing, meaning the measured intensity I of the x-ray radiation as a function of the image coordinate (x, y), normally exist with a resolution (for example 4096 intensity levels) that cannot be used in the reproduction medium, for example a monitor with 256 grey levels.
In order to achieve as optimal an image display as possible, these intensity values must be mapped to the grayscale values that can be displayed on the reproduction medium, the mapping occurring with suitable transformation rules (lookup characteristic lines or lookup tables). In other words: the grayscale values used for the image display are associated with the intensities belonging to the individual image points. In known x-ray apparatuses, the entire x-ray image is always evaluated for the determination of these greyscale values since in particular the presence or the position of direct radiation regions in the x-ray image is not known in advance. This has the result that the grayscale values in the subject image region are no longer displayed with the best possible contrast resolution of the monitor (i.e. no longer utilizing the maximum possible grayscale value or grey level with, for example, 256 grayscale value), but rather are displayed with few grayscale values since, in the transformation, the extreme values (diaphragm region and direct radiation region) of the intensities measured by the x-ray receiver significantly limit the grayscale value range remaining for the subject image region in the transformation.
Moreover, direct radiation regions present in the x-ray image can visually “overload” the observer. The contrast resolution capability of the eye is thereby reduced and fine contrast details in the subject image region are also not detectable when they are reproduced on a monitor. Moreover, such over-intensity is always uncomfortable and should be generally avoided for this reason.
The possibility does exist in principle to minimize the direct radiation regions using diaphragms (X-iris diaphragms or filter diaphragms). In practice, however, this possibility is frequently not used since a correct setting of the diaphragms is time-consuming, particularly in the acquisition of a number of x-ray exposures from different directions. Moreover, due to the complex geometric shape of the examination subject, such a setting of diaphragms preventing direct radiation regions is not possible in all cases without loss of diagnostic information.